Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a program.
Description of the Related Art
Optical interference tomography apparatuses based on optical coherence tomography (OCT) utilizing multi-wavelength light wave interference are used in ophthalmology. Optical interference tomography apparatuses apply measuring light, which is low-coherent light, to an eye to be examined, and measure reflected scattered light from the eye to be examined by utilizing an interference system. By scanning the measuring light over the eye to be examined, a tomographic image of the eye to be examined can be captured with high resolution.
Various noises are generated in a tomographic image of an eye to be examined captured with such an optical interference tomography apparatus. A noise reduction is achieved by performing averaging processing on plural tomographic images captured for the same region. In addition, because involuntary eye movement of an eye to be examined prevents photographing of exactly the same region, as preprocessing to be performed for averaging processing on plural tomographic images, detection of a positional difference between the plural tomographic images and correction of the positional difference by translation and rotation of all the tomographic images, are performed. Detection of a positional difference will be explained below in detail. Calculating similarities (or differences) among plural tomographic images using pattern matching and obtaining the positional relationship between the tomographic images having the highest similarity (or lowest difference), is a general method for detecting a positional difference.
An ophthalmological photographing apparatus, which provides a high-definition tomographic image by performing high-accuracy detection of a positional difference and high-accuracy correction of the positional difference, has been suggested. In a technique disclosed in Japanese Patent No. 5199031, a tomographic image is divided into plural regions and pattern matching is performed for each of the plural regions. Accordingly, detection of a positional difference and correction of the positional difference can be performed for each divided region, and an excellent tomographic image can be obtained for which a local positional difference within a single tomographic image has been coped with.
An eye to be examined contains a macular portion and an optic disk rim portion. When such an eye to be examined is photographed with an optical interference tomography apparatus, pulsation of blood vessels, such as constriction and dilation, causes the image brightness of a blood vessel portion in a tomographic image to change. Further, since the optic disk rim portion is a portion where the incidence angle of measuring light relative to a target applied from the optical interference tomography apparatus is large, reflected scattered light which is directed from the eye to be examined towards the optical interference tomography apparatus is weak. Accordingly, the image brightness of the optic disk rim portion in the tomographic image is low and susceptible to noise.
A case where, in order to reduce noise, averaging processing is performed on plural tomographic images captured for the same region including such a blood vessel portion or optic disk rim portion, will be discussed below.
Regarding portions other than the blood vessel portion and the optic disk rim portion in a tomographic image, in the case where there is a positional difference between tomographic images, the similarity calculated by pattern matching is low. Meanwhile, in the case where there is no positional difference between tomographic images, the similarity calculated by pattern matching is high.
In contrast, regarding the blood vessel portion and the optic disk rim portion, since these portions are susceptible to a brightness change and noise, even if there is no positional difference between tomographic images, the similarity is low. It is therefore difficult to distinguish between the case where there is a positional difference and the case where there is no positional difference.
Although there is no case where only the blood vessel portion or the optic disk rim portion exists in a template, there are many cases where such a portion is included in a template. In such cases, similarities calculated by pattern matching are somehow influenced by the reduction in similarity attributable to the blood vessel portion or the optic disk rim portion. The influence of the reduction in similarity depends on the proportion of the blood vessel portion or the optic disk rim portion in a region in a template.
As a result, a tomographic image becomes blurred by performing averaging processing in a state where the number of tomographic images to be subjected to averaging processing is not increased or a state where a positional difference between tomographic images is not correctly detected.
In the case where a template is divided into plural regions and a positional difference is detected, as in Japanese Patent No. 5199031, the division into the plural regions generates a region with a large proportion of the blood vessel portion or the optic disk rim portion in the divided region. Therefore, a situation occurs frequently in which the number of tomographic images on which averaging processing has been performed is small or a tomographic image partially blurs.